Many enclosures that contain sensitive instrumentation must maintain very clean environments in order to operate properly. Examples include the following: enclosures with sensitive optical surfaces, or electronic connections that are sensitive to particulates and gaseous contaminants which can interfere with mechanical, optical, or electrical operation; data recording devices, such as computer hard disk drives that are sensitive to particles, organic vapors, and corrosive vapors; enclosures for processing, transport or storage of thin films and semiconductor wafers; and electronic control boxes such as those used in automobiles and industrial applications that can be sensitive to particles, moisture buildup, and corrosion as well as contamination from fluids and vapors. Contamination in such enclosures originates from both inside and outside the enclosures. For example, in computer hard drives, damage may result from external contaminants as well as from particles and outgassing generated from internal sources. The terms "hard drives" or "hard disk drives" or "disk drives" or "drives" will be used herein for convenience and are understood to include any of the enclosures mentioned above.
One serious contamination-related failure mechanism in computer disk drives is static friction or "stiction". Stiction results from the increased adhesion of a drive head to a disk while the disk is stationary plus increased viscous drag parallel to the head-disk interface. Newer high density disks are more sensitive to contamination-caused stiction because they are smoother and only thin layers of lubricants are present. Contaminants on the disk change the surface energy and the adhesive forces between the head and disk, which cause stiction. Also, vapors that condense in the gap between the head and disk can cause stiction. In addition to increasing power requirements for spinning up the drive, stiction forces can cause permanent mechanical deformation of the head suspension and gimbal assemblies. The latter have become extremely sensitive to small forces with the introduction of smaller air bearing sliders with lower applied loads, using thinner structural elements in the suspension and gimbal to maintain slider flying heights and attitudes with tremendous precision. Further exacerbating these effects are the newer lower energy, lower torque motors being used in smaller disk drives for portable computers.
Condensation of volatile organic contaminants (VOCs) onto magnetic head and disk surfaces will increase the physical head-medium separation, which will result in signal loss and increased data errors. This is increasingly true as linear bit density continues to rise at a breakneck pace. Accumulation of heavy VOCs on the critical air bearing surfaces of the head slider, through transfer from the much larger disk surface, can cause destabilization of the flying dynamics of the slider as well as significant signal losses. Interaction of VOCs and disk lubricant can lead to chemical degradation of the lubricant, especially during instances of head-disk contact and the accompanying elevated temperatures. This can lead to build-up on the disk surface of viscous, high molecular weight reaction products which are unable to replenish localized regions of lubricant depletion. Other degradation products may be volatile and permanently escape the head-disk interface. These processes can eventually result in elevated wear of head or disk surfaces, increased stiction, higher error rates, and ultimately reduced lifetime for the drive.
Acid gases, which are widely present in pollution and smog as well as industrial environments, can have especially harmful effects on drive reliability if allowed to circulate inside the drive. These compounds will adsorb onto head and disk surfaces and corrode exposed metallic layers via galvanic reaction in the presence of water. They can also be transported by moisture through pinholes in protective coatings on the head and disk. Corrosion typically results in loss of magnetic properties, as well as accumulation of reaction products on critical surfaces.
Another serious contamination-related failure mechanism in computer disk drives is head crashes. Head crashes can occur when particles get into the head disk interface. Newer high density drives have 30 nanometer or less flying heights or spacing between the head and disk during operation and typically have disks rotating 5400 revolutions per minute or greater. Even submicron-sized particles can be a problem, causing the head to crash into the particle or the disk after flying over a particle, bringing the drive to an abrupt failure mode. Particles which do not cause a head crash may still adversely affect data integrity and mechanical reliability of a drive. Small, hard inorganic particles can cause wear of a head or disk, which may result in permanent signal loss, degradation of protective coatings, or further debris generation. Wear of carbon overcoats on the head or disk can accelerate corrosion of sensitive layers through the action of moisture, acid gas contaminants, and elevated interface temperatures. Plowing of small hard particles into the disk surface can create scratches, asperities, or pile-ups of disk material. Current magnetic head technologies, employing magnetoresistive (MR) elements for sensing of magnetic flux emanating from the disk, are highly sensitive to transient temperature excursions as caused by interaction of the head with asperities or adhered particles on the rapidly moving disk. The resulting change in resistance of the MR element may be misinterpreted as magnetic signal, causing data errors. This phenomenon is well known in the industry and is referred to as a thermal asperity.
In addition, disk drives must be protected against a large number of contaminants in the surrounding environment that can penetrate the drive. This is true for drives used in small to medium sized computer systems which may not be used in the typical data processing environment and is especially true in drives that are removable and portable to any environment such as disk drives that are used in laptop computers or in Personal Computer Memory Card International Association (PCMCIA) slots.
Filtration devices to keep particles from entering these enclosures are well known. They may consist of a filtration media held in place by a housing of polycarbonate, acrylonitrile butadiene styrene (ABS), or some other material; or they may consist of a filtration media in the form of a self-adhesive disk utilizing a layer or layers of pressure sensitive adhesive. These devices are mounted and sealed over a vent hole in the enclosure to filter particulates from the air entering the drive. Filtration performance depends not only on the filter having a high filtration efficiency but also on having a low resistance to air flow so that unfiltered air does not leak into the enclosure through a gasket or seam instead of entering through the filter. Such filters work well for particulates of external origin, but do not address the problems from vapor phase contaminants.
Combination adsorbent breather filters to keep particulates and vapors from entering enclosures are also well known. These can be made by filling a cartridge of polycarbonate, ABS, or similar material with adsorbent and securing filter media on both ends of the cartridge. Examples of such filters are described in U.S. Pat. No. 4,863,499 issued to Osendorf (an anti-diffusion chemical breather assembly for disk drives with filter media having a layer impregnated with activated charcoal granules); U.S. Pat. No. 5,030,260 issued to Beck et al. (a disk drive breather filter including an assembly with an extended diffusion path); U.S. Pat. No. 5,124,856 issued to Brown et al. (a unitary filter medium with impregnated activated carbon filters to protect against organic and corrosive pollutants); and U.S. Pat. No. 5,447,695 issued to Brown et al. (Chemical Breather Filter Assembly). Unfortunately, many of these designs are too large and take up too much space in today's miniaturized drives. They again filter only incoming air of particles and mainly incoming air of vaporous contaminants, although some internal air can also be cleaned from internally generated vaporous contaminants since the filters are inside the drive and these contaminants will diffuse into the adsorbent sections of the filters. None of these filters address cleaning the air of internal particles.
A second combination adsorbent breather filter is also well known that encapsulates the adsorbent material such as an impregnated activated carbon polytetrafluoroethylene (PTFE) composite layer between two layers of filter media and is applied over a hole in the enclosure with a layer of pressure sensitive adhesive. These filters work well and are of a size that can be used in today's small drives but are typically designed to filter air coming into the drive. Thus, the adsorbent is typically primarily desired to adsorb both organic and corrosive vapors from the outside environment and will filter particulates only from air coming into or leaving the drive. Internally generated vapors can be adsorbed by these filters, but often times they are used in conjunction with another internal adsorbent so they can be smaller in size; therefore, such filters do not contain enough adsorbent to adequately adsorb all the internally generated contaminants. Again, particles are also generated inside the drive and are not typically captured by these filters.
A diffusion tube can be included with either the particulate breather filter or an adsorbent breather filter as described in U.S. Pat. No. 5,417,743 by Dauber. Diffusion tubes provide additional protection against vaporous contaminants (including moisture) entering the drive through the breather opening by providing a diffusion barrier in the form of the diffusion tube which creates a tortuous or a longer path for air to travel before entering the drive enclosure. Diffusion tubes reduce the number of contaminants reaching the interior of the enclosure (and/or the adsorbent depending on the location of the filter) and increase the diffusion time constants or time required to reach chemical equilibrium with the environment. As used herein, for convenience, the term "diffusion tube" may refer to either a conventional tortuous path or it may refer to a non-tortuous cavity into which incoming air passes before entering the filter.
Internal particulate filters, or recirculation filters, are also well known. These filters are typically pieces of filter media, such as expanded PTFE membrane laminated to a polyester nonwoven backing material, or "pillow-shaped" filters containing electret (i.e., electrostatic) filter media. They are pressure fit into slots or "C" channels and are placed in the active air stream such as near the rotating disks in a computer hard disk drive or in front of a fan in electronic control cabinets, etc. Alternatively, the recirculation filter media can be framed in a plastic frame. These filters work well for removal of internally generated particles but do not address the problem of vapor phase contaminants, nor do they provide ultimate protection from external particles entering the drive.
Internal adsorbent filters are also well known. One example is described in U.S. Pat. No. 4,830,643 issued to Sassa et al. This patent teaches an adsorbent filter where a powdered, granular or beaded adsorbent or adsorbent mixture is encapsulated in an outer expanded PTFE tube. This filter is manufactured by W. L. Gore & Associates, Inc., Elkton, Md., and is commercially available under the trademark GORE-SORBER.RTM. module. While this is highly effective at collecting vapor phase contaminants, it is currently only available in large and medium sizes like filter volumes down to about 3 cc. In its present form, this filter is incapable of fully addressing the growing needs for even smaller and more compact adsorbent filters, nor is it designed to filter the internal air of particulate contamination. A second well known internal adsorbent assembly incorporates a layer of adsorbent, such as activated carbon/PTFE composite, between an encapsulating filter layer and layer of pressure sensitive adhesive that helps encapsulate the adsorbent as well as provides a means of mounting the adsorbent assembly on an interior wall in the enclosure. Such a filter is described in U.S. Pat. No. 5,593,482 issued to Dauber et al. Again neither of these filters addresses particulate contaminants. A third internal adsorbent assembly incorporates a layer of adsorbent such as activated carbon/PTFE composite between two layers of filter media or is alternately wrapped in a layer of filter media and can be installed between slots or "C" channels much the way a recirculation filter is installed but without much real airflow through the filter. Such a filter is described in U.S. Pat. No. 5,500,038 issued to Dauber et al., and, as with the other filters mentioned, this construction does not provide significant particle capturing capability.
As stated above, all of these internal adsorbent filters work well at adsorbing vapor phase contaminants, but they do not filter particulates very well. They can collect particles by some impaction of particles onto the filter (i.e., by having the larger particles impacting or colliding with the adsorbent filter as particle-laden air speeds around the filters) or by diffusion of particles onto the filter. However, these filters do not perform nearly as well as standard recirculation filters that work by a combination of sieving (mechanically capturing particles too large to pass through the pore structure of the filter), impaction (capturing particles too large to follow the bending air streams around filters or the fibers of the filter), interception (capturing particles that tend to follow the air streams, but are large enough to still intercept a filter fiber or in other words those particles with a diameter equal to or less than the distance between the fiber and the air stream line), and diffusion (capturing smaller particles buffeted about by air molecules in a random pattern and coming into contact with a filter fiber to become collected).
A commercially available adsorbent recirculation filter, available from The Donaldson Company, incorporates activated carbon beads glued to a nonwoven carrier that is sandwiched between two layers of electret filter material and two layers of plastic support screen. This filter provides some adsorbent protection at the sacrifice of some internal particle filtration effectiveness, as this construction appears to increase resistance to air flow through the filter relative to a conventional recirculation filter. The adsorbent capability is limited, however, due to, for example, the constraints of the filter size and the blockage of adsorbent surface area by the glue holding the carbon to the carrier. Moreover, this filter does not filter particles from air entering the drive.
Another issue in today's drives is contamination due to corrosive ions such as chlorine and sulfur dioxide. To adsorb these compounds the adsorbent is typically treated with a salt to chemisorb the contaminants. When the filters described in the preceding paragraph were washed in deionized water, large amounts of these salts were released, making it unacceptable to today's sensitive disk drive environments. An alternative washable adsorbent recirculation filter is described in U.S. Pat. No. 5,538,545 issued to Dauber et al., wherein expanded PTFE membranes or other hydrophobic materials are used to encapsulate the adsorbent. However, these filters still do not filter air as it comes into the drive before it has had a chance to deposit particles and do damage to the drive.
Combinations of several filters having different functions in a single drive have been taught. For example, U.S. Pat. No. 5,406,431, to Beecroft, describes a filter system for a disk drive that includes an adsorbent breather and recirculation filter in specifically identified locations within the drive. Also, U.S. Pat. No. 4,633,349, by Beck et al., teaches a disk drive filter assembly comprising a dual media drum type filter element in a recirculating filter assembly that surrounds a breather filter. Further, U.S. Pat. No. 4,857,087, to Bolton et al., teaches incorporating a breather filter in a recirculation filter housing, but has significantly more parts and incorporates a third filter element complete with housings, apertures, and gaskets to accomplish this inclusion. The combinations described in these patents either locate the filter components in separate regions of the disk drive or incorporate space-consuming fixtures to orient the component parts within the drives.
As disk drives have become smaller and the prices have declined, there has been a push for simplification and the reduction in the number of parts in a drive to reduce cost and improve performance. Also, as the drives continue to increase in recording data density and capacity, they continue to become more sensitive to particulate and vaporous contamination, such that the existing filtration means often do not meet these ever more demanding filtration requirements.
Accordingly, a primary purpose of the present invention is to provide an improved rigid multiple function part that can filter both incoming (i.e., external to the enclosure) air and internal recirculating air of particulates.
A further purpose of the present invention is to provide an improved rigid multiple function part that can filter both incoming and internal recirculating air of both particulates and vapor phase contaminants.
A further purpose of the present invention is to provide a rigid multiple function part, as described above, which further incorporates a diffusion tube.
A further purpose of the present invention is to provide a rigid multiple function part, as described above, which further incorporates a gasket to help to seal the disk drive housing.